(1) Field of the Invention
The present invention relates to light emitting diode ("LED") traffic signals, and more particularly, to a circuit for interfacing between the load switches and associated conflict monitors used in traditional traffic controllers and the LEDs used in such signals.
(2) Background Information
Traditionally, traffic signals have used incandescent lamps to generate the light signals used to control traffic. One problem with incandescent traffic signals is the high maintenance costs associated with replacing the incandescent lamps that have a much shorter life cycle than the LEDs.
An alternative to incandescent traffic signals are traffic signals which use light emitting diodes (LEDs). Although LED traffic signals are very reliable in comparison to traditional incandescent traffic signals, the use of LED's in traffic signals raises other problems which must be solved.
One problem with LED traffic signals is the type of electrical load they present to utility companies' A.C. power lines. The manner in which electrical devices use current is of great interest to utility companies because of the impact such devices can have on their ability to efficiently deliver energy to customers. Such use is often of interest to customers as well, since commercial customers can receive reductions in their electric bills in the form of Demand Side Rebates when they provide the type of electrical load a utility company considers to be desirable.
A traditional incandescent traffic signal is a desirable type of electrical load because it is inherently a resistive load that uses current linearly, and as such, allows a smooth transfer of power from the incoming line to the traffic signal load.
In contrast, an LED traffic signal typically includes a power supply for the arrays of LEDs that replace the individual incandescent lamps in traditional traffic signals. The rectified and filtered front end of such a power supply uses current in a manner that is very different from resistive loads and, thus, undesirable to utility companies.
Another concern with traffic signals that use LEDs is the output intensity of the LED arrays that replace the incandescent lamps. LEDs are inherently current mode devices. The light output of an LED is linearly related to the current passing through it, albeit with some heating effects at higher currents. As such, it is important that the LEDs be powered by a well regulated current source, since the degree of regulation directly affects the consistency of output intensity of the lamp as a whole.
Turn-off of the LED arrays can also be a significant problem in LED traffic signals because of the extremely small amount of current required to produce a noticeable glow in LEDs, particularly at night.
A further concern that arises out of the use of LEDs in traffic signals is the need to verify that two signal lights are not on at the same time. LED arrays replacing older incandescent lamps must be compatible with the older incandescent load switch technology and its associated conflict monitors. A conventional monitor, which senses voltage across a load, such as an incandescent lamp, has preset thresholds to determine whether or not the signal light is really on or off. These monitors were designed to operate with incandescent lamps, which have essentially constant input impedance once up to operating temperature (this is exactly what defines an incandescent signal as being a resistive load). In contrast, the power supplies and LEDs in newer traffic signals have more of a "step" turn-on characteristic, which leads to a high input impedance when not fully turned on. Without modification, LED signals look to the conventional conflict monitors as if they never turned off, falsely indicating a load switch failure.
An additional concern regarding conflict monitor compatibility is the failed state of any traffic signal. Traditional monitors sense voltage across a signal, and, as such, are only useful with incandescent signals, unless specific modifications are made to other types of signals. Unless specifically designed to accommodate this situation, an LED type signal would not satisfy the compatibility requirement, since the conflict monitor would not sense the failure of the LED signal.
Another concern in LED traffic signals is the circuit arrangement of the LEDs which replace the traditional incandescent lamps. Whether the LEDs are connected in parallel or series, provision must be made to minimize the loss of light output due to any individual catastrophic LED failures.